1. Field of the Invention
The present invention relates to a multiple-pass laser amplifier system, and more particularly to a method and apparatus for efficiently amplifying light. The present invention further relates to a method and structure for passing a beam of light through an optical component, such as a laser amplifier, multiple (at least four) times in a collinear fashion without using active optical switching.
2. Background
Laser systems of the oscillator/amplifier type, i.e., a laser oscillator which generates a laser pulse which is then amplified to the desired energy level in one or more amplifier units, are known. Where the signal entering the amplifier is small, the problem of efficient energy extraction can be significant. Amplification of small laser signals can be performed by a linear chain of amplifier units; however, the energy extraction efficiency of such a system is low, since saturation of the laser gain typically only occurs in the last amplification unit. Energy extraction efficiency may be improved by passing the laser beam twice through each amplifier stage, using a double-pass collinear geometry where a polarizer and quarter-wave retardation plate are used to separate the reflected beam from the incoming beam. Such a system generally employs a polarizer and quarter-wave plate arranged as shown in FIG. 1. Assuming the beam incoming to the polarizer from the left is plane polarized, with the plane of polarization such that the beam is passed by the polarizing element, then the beam passes through the quarter-wave plate, which is adjusted in such a way as to convert the polarization of the laser beam to be circular. The beam then passes through the amplifier, reflects off the mirror, passes through the amplifier for a second time and is incident on the wave plate for a second time. The rotation of the polarization vectors of the incoming beam is such that the beam passes out the wave plate with plane polarization in a direction orthogonal to the polarization of the original beam. This geometry allows collinear multiple passing of the amplifier but is limited to only two passes. Also see Brueckner, et al., U.S. Pat. No. 4,019,151, which use a double pass collinear geometry plus a delayed time sequence for more efficient amplification.
An alternative system, which includes multiple passing of a beam through an amplifier unit in order to increase the energy extraction efficiency, is the use of a regenerative amplifier, where the laser amplifier is placed within an optical cavity and the optical signal to be amplified is switched into the cavity by an active control means, and maintained within the cavity until such time as the desired energy level is attained at which point the signal is switched out of the cavity, for example, by a Pockels cell. See, for example, W. H. Lowdermilk, et al., The Multipass Amplifier: Theory And Numerical Analysis, J.Appl. Phys. 51(5), May 1980, and J.E. Murray, et al., ND:YAG Regenerative Amplifier, J.Appl. Phys. 51(7), Jul. 1980. The regenerative amplifier has the advantage over the oscillator/amplifier designs in that only one amplifier stage is required to bring the signal to a saturation level. On the other hand, the regenerative amplifier requires active optical switching to trap the pulse within the optical cavity and to dump the signal from the cavity. Additionally, the length of the regenerative amplifier cavity is set by the duration of the laser pulse to be amplified and the speed of the active optical switches. The minimum length of the regenerative amplifier cavity is limited to being greater than half of the length of the optical signal plus the optical length associated with the activation time of the optical switching.